The Pseudomonas syringae effector protein HopZ1a suppresses effector‐triggered immunity

Abstract

*The Pseudomonas syringae pv syringae type III effector HopZ1a is a member of the HopZ effector family of cysteine-proteases that triggers immunity in Arabidopsis. This immunity is dependent on HopZ1a cysteine-protease activity, and independent of known resistance genes. We have previously shown that HopZ1a-triggered immunity is partially additive to that triggered by AvrRpt2. These partially additive effects could be caused by at least two mechanisms: their signalling pathways share a common element(s), or one effector interferes with the response triggered by the other. *Here, we investigate the molecular basis for the partially additive effect displayed by AvrRpt2- and HopZ1a-triggered immunities, by analysing competitive indices, hypersensitive response and symptom induction, PR-1 accumulation, expression of PR genes, and systemic acquired resistance (SAR) induction. *Partially additive effects between these defence responses require HopZ1a cysteine-protease activity, and also take place between HopZ1a and AvrRps4 or AvrRpm1-triggered responses. We establish that HopZ1a-triggered immunity is independent of salicylic acid (SA), EDS1, jasmonic acid (JA) and ethylene (ET)-dependent pathways, and show that HopZ1a suppresses the induction of PR-1 and PR-5 associated with P. syringae pv tomato (Pto)-triggered effector-triggered immunity (ETI)-like defences, AvrRpt2-triggered immunity, and Pto or Pto (avrRpt2) activation of SAR, and that suppression requires HopZ1a cysteine-protease activity. *Our results indicate that HopZ1a triggers an unusual resistance independent of known pathways and suppresses SA and EDS1-dependent resistance.

Insights into plant immunity signaling

“…These results, in addition to other phenotypic and molecular assays, allowed us to establish that HopZ1a suppresses ETI. In this report, we were also able to detect a reduction in the defence response triggered by AvrRpt2 in eds1 mutant plants, 14 revealing the quantitative contribution of EDS1 to RPS2-mediated signaling. It also mediate ETI and trigger an HR.…”

“…13 In a recent report, we have also used competitive index assays to analyse functional relationships between T3SEs within the plant cell, showing that HopZ1a triggers a partially additive plant defence response to that triggered by the well characterized effectors AvrRpt2, AvrRpm1 and AvrRps4. 14 In addition, we applied CI assays to rule out the hypothesis of these partially additive defences being due to the signaling pathways sharing common elements, demonstrating that HopZ1-triggered immunity is indepedent of SA, EDS1, jasmonic acid (JA) and ethyene (ET)-dependent pathways. These results, in addition to other phenotypic and molecular assays, allowed us to establish that HopZ1a suppresses ETI.…”

Analysis of the ZAR1 Immune Complex Reveals Determinants for Immunity and Molecular Interactions

“…4). The downstream pathway activated by AtZAR1 in response to HopZ1a is still uncharacterized and does not involve any of the classic ETI regulators identified so far (Lewis et al, 2010;Macho et al, 2010). Our identification of an autoactive form of AtZAR1 will help to identify new components of this signaling pathway.…”

Non-Recognition-of-BTH4, an Arabidopsis Mediator Subunit Homolog, Is Necessary for Development and Response to Salicylic Acid

“…P. syringae pv maculicola CR299 has been described (Ritter and Dangl, 1995). SAR was measured as reported (Macho et al, 2010), inoculating leaves with Pto(avrRpm1) or a mock treatment using a blunt syringe. For all the experiments, at least three independent treatments were performed (three independent sets of plants sown and treated on different dates).…”